1. Field of the Invention
The present invention relates to an MTJ cell, and particularly to a yoke-type MTJ cell and manufacturing method thereof, and to an MTJ-based MRAM device.
2. Description of the Related Art
Magnetoresistive random-access memory (MRAM) devices configured of magnetic tunnel junctions (MTJ) use magnetized states of cells to store data. Hence, unlike DRAM and similar memory devices, MRAM devices are nonvolatile, meaning that they preserve the storage state even when the power supply is switched off. In addition to this feature, MRAM devices show potential for high-speed random-access times (a few nanoseconds), similar to those of SRAM, as well as a large storage capacity on par with DRAM. Consequently, MRAM devices have become a popular subject of research and development.
As is well known, an MTJ has three stacked layers, including a ferromagnetic pinned layer, a nonmagnetic barrier layer, and a ferromagnetic storage layer (hereinafter referred to simply as the “pinned layer,” “barrier layer,” and “storage layer”). While the magnetization direction of the pinned layer is fixed, magnetization of the storage layer can be changed between a parallel and antiparallel orientation with the magnetization of the pinned layer, greatly changing the electrical resistance in the stacked direction of the MTJ between a small and large resistance. The MRAM device uses the magnitude of this resistance as the storage state for one bit worth of data.
A write operation on the MTJ is performed by passing a write current of prescribed magnitude in one direction or the other through a conductive line contacting the storage layer of the MTJ. The write current produces an induced magnetic field near the MTJ modifying the direction of magnetization in the storage layer. A read operation is performed by detecting the magnitude of resistance in the MTJ when a current flows between the conductive line and pinned layer, for example.
However, unlike with DRAM and other memory types, it is presently not possible to reduce the write current for an MTJ to a few milliamperes (mA) or less when the MTJ is reduced below a certain size. This is a first problem related to MTJs and occurs because the coercivity of a ferromagnet increases as it is reduced in size. This leads to an increase in the magnetic field strength required to reverse its magnetization, i.e., an increase in the write current needed to induce the magnetic field.
Given that DRAM and other circuits commonly handle currents of several hundred microamperes (μA) at most, employing MTJs with write currents of around ten times that magnitude in an MRAM device would cause serious strain on the peripheral circuits of the device, and the write lines in particular. Thus, monolithic integration of MTJs with peripheral circuits using high-definition CMOS technology, for example, would be highly difficult to implement.
A second problem with MTJs is their sensitivity to strong external magnetic fields. Since an MTJ stores data based on the difference between the magnetization of the storage layer and pinned layer, proper storage and proper read/write operations become impossible when a strong external magnetic field changes the direction of magnetization in the storage layer or in both the storage layer and the pinned layer.
A third problem associated with MTJs arises in MRAM devices formed through large-scale integration of MTJs. Namely, the induced magnetic field generated by the write current for a specific MTJ may leak and influence adjacent MTJs, leading to potential false writes.
A yoke-type MTJ has recently been proposed to resolve these problems. The yoke-type MTJ has a ferromagnetic layer or high-permeability magnetic layer called a “yoke” for covering the conductive line that contacts and supplies a write current to the storage layer of the MTJ so that the conductive line is surrounded by the storage layer and yoke. Patent Documents 1-3 indicated below describe examples of yoke-type MTJs.
The storage layer and yoke surrounding the conductive line form a closed magnetic path in the yoke-type MTJ. As a result, the magnetic field strength generated in the storage layer when the write current flows through the conductive line is roughly twice that of a yoke-less MTJ. In other words, the write current for producing a comparable magnetic field strength needed to reverse magnetization can be halved.
However, even this MTJ configuration has not been enough to reduce write current values to an order of mA or less, making it fundamentally difficult to resolve the above-mentioned first problem. However, the yoke-type MTJ does offer a solution to the second and third problems, since the yoke produces a magnetic shielding effect.